Nuclear radiation detector and method of manufacturing same

ABSTRACT

A method of manufacturing a nuclear radiation detector comprising forming a plate from a diamond crystal whose thickness does not exceed the distance travelled by charge carriers within the plate under the influence of an applied field. The plate is then annealed in a vacuum at a temperature of 1,000* to 1,300*C whereafter blocking and injecting contacts are formed on opposite sides of the plate. During operation of the detector, polarization within the plate is removed by injection of charge carriers into the plate via the injecting contact, the latter being formed on the side of the plate opposite to that which is subjected to the radiation to be detected.

United States Patent [191 Kozlov et al.

[451 July 23, 1974 NUCLEAR RADIATION DETECTOR AND METHOD OFMANUFACTURING SAME [73] Assignee: Ordena Levina Fizichesky InstitutImeni P. N. Lebedeva, Leninsky Prospekt, Moscow, USSR.

[22] Filed: July 8, 1971 [21] Appl. No.: 160,917

Related US. Application Data [62] Division of $61. No. 716,953, March28, 1968, Pat.

[56] References Cited UNITED STATES PATENTS 8/1956 Youmans 250/833 R6/1968 I-Ieiman 29/571 9/1970 Oosthoek et al 250/833 R FOREIGN PATENTSOR APPLICATIONS 1,179,303 10/1964 Germany 317/235 OTHER PUBLICATIONSElectronic Properties of Diamonds, Champion, 1963, pp 36 and 91 (copy inScientific Library TS 753 C45) Primary Examiner-Charles W. LanhamAssistant Examiner-James W. Davie Attorney, Agent, or Firm-Waters,Roditi & Schwartz [5 7] ABSTRACT A method of manufacturing a nuclearradiation detector comprising forming a plate from a diamond crystalwhose thickness does not exceed the distance travelled by chargecarriers within the plate under the influence of an applied field. Theplate is then annealed in a vacuum at a temperature of 1,000 to'l,300Cwhereafter blocking and injecting contacts are formed on opposite sidesof the plate. During operation of the detector, polarization within theplate is removed by injection of charge carriers into the plate via theinjecting contact, the latter being formed on the side of the plateopposite to that which is subjected to the radiation to be detected.

11 Claims, 2 Drawing Figures NUCLEAR RADIATION DETECTOR AND METHOD OFMANUFACTURING SAME CROSS RELATED APPLICATION This application is adivision of Ser. No. 716,953 filed Mar. 28, 1968, and issued as US. Pat.No. 3,665,193 on May 23, 1972.

The present invention relates to methods of manufacturing nuclearradiation detectors.

There are known nuclear radiation detectors consisting of a crystal ofnatural diamond with a lower nitrogen content (the nitrogenconcentration is usually less than atoms cm provided with electriccontacts. When applying a potential difference across the diamond andirradiating it with nuclear particles from any side, current pulses areinduced inside the crystal. These currentpulses produce, in an externalcircuit, voltage pulses which are amplified and counted by appropriateapparatus.

This behavior is shown only by a small number of diamond crystals andthe counting properties of such detectors are diverse anduncontrollable. Such detectors have poor counting efficiency and lowenergy resolving power, and operate with incomplete collection of thecharge carriers created in the crystal by incident nuclear particles. Inaddition, electric polarization occurs in these crystals, since theresistivity of said crystals is high As a result, their countingproperties deteriorate under irradiation. Known methods of removingpolarization by heating or illumination with light of appropriatewavelengths are inconvenient and ineffective. For these reasons, thedetectors based on diamond crystals have not found wide practicalapplication.

It is an object of the present invention to provide a method formanufacturing a diamond detector which can operate at room and highertemperatures and possess good counting efficiency, complete chargecollection and high energy resolution, as well as constancy of itsproperties underprolonged irradiation, said detector being made from ahigh resistivity diamond crystal and, hence, operating withoutincreasing noise level.

In the accomplishment of the above and other ob jects of the invention,in a detector consisting of a diamond crystal plate with two electriccontacts located at its opposite sides across which plate a potentialdifference is applied, according to the invention, the thickness of theoperating range of the plate between the contacts is kept at or belowthe distance traveled under the influence of the applied electric fieldby the charge carriers created by nuclear radiation in the diamondcrystal. The contact through which the incident nuclear particlespenetrate into the crystal is made blocking in relation to the chargecarriers, while the opposite contact is made from a material capable, inconjunction with diamond, of injecting the charge carriers into thecrystal under the influence of the electric field.

If the detector is made from diamond in which the distance traveled byelectrons is longer than that traveled by holes, the contact on theunirradiated side of the plate should inject holes and the positivepotential is applied to this contact. If the detector is made fromdiamond in which the distance traveled by electrons is less than thattraveled by holes, the contact on the unirradiated side of the plateshould inject electrons and the negative potential is applied to thiscontact.

When the. condition of complete charge collection is observed andprovided that the thickness of the diamond crystal plate is low, arecess is made in the thick plate of the crystal with a view toincreasing the mechanical strength of the detector the thickness of thebottom of said recess being equal to the distance traveled by the chargecarriers.

As experiments have shown, silver, gold, platinum and graphite may beused as a material insuring, in con, junction with diamond, a contactinjecting holes. Such a contact may also be provided by the surfacelayer of the diamond drystal plate doped with aluminum or boron.

Graphite may serve as a material insuring, in conjunction with diamond,a contact injecting electrons.

This contact may also be provided bythe surface layer of the diamondcrystal plate dopedwith phosphorus, lithium or carbon.

The material providing a blocking contact may be gold, silver orplatinum. Such a contact may also be provided by the graphitized surfaceof the diamond crystal plate and by doping the surface layer of theplate with boron, aluminum, phosphorus, lithium and carbon. Theformation of the blocking and injecting contacts is secured not only byusing said materials, but also by applying to it a potential ofappropriate polarity, as well as owing to the damage of the surfacecrystalline structure of the plate, such as in the case ofgraphitization or doping.

The detector may be manufactured by a method wherein, according to theinvention, a plate is cut from a diamond crystal, the thickness of theplate being equal to the distance traveled by the charge carriers in thecrystal. With a view to prolonging the lifetime of the carriers, saidplate is annealed in vacuum at l,O00 to 1,300C. Prior to forming thecontacts, the annealed plate of diamond crystal is etched by heating inan oxygen-containing medium in order to reduce the-rate of surfacerecombination of the charge carriers, if necessary.

For forming the blocking and injecting contacts, both sides of thecrystal plate are covered with a paint of silver, gold or platinum, andthe plate is heated to a temperature of 500 to 700C. The plate is heldat this temperature for 2 to 3 hours in order to burn the metal into theplate.

The blocking and injecting contacts can be formed by applying to bothsides of the crystal plate a solution of gold, silver or platinum saltsand by heating the plate to a temperature of 500 to 700C for severalminutes in order to restore the metal.

For forming a graphite contact for injecting both electrons and holes,one side of the diamond crystal plate is covered with a colloidalgraphite suspension, and the plate is heated in vacuum to a temperatureof 500 to 600C for about 3 hours.

A blocking contact is obtained by evaporating a film of gold, silver orplatinum over one side of the plate. In some cases, for forming ablocking contact, the diamond crystal plate is graphitized by heating itin a vacuum of 0.1 torr for about 30 min at a temperature of l,000 to1,300C.

For a better understanding of the invention, there is presentedhereinbelow a description of an exemplary embodiment thereof withreference to the accompanying drawings, wherein:

FIG. 1 is a diagrammatic elevation view which shows the detector inaccordance with the invention; and

l e .3 FIG. 2 shows a modification in which the detector of theinventionconsist of a diamond crystal plate which a recess, providedwith contacts. V

Thedetectorof the invention (FIG. 1) consistsofa diamond crystal plate 1on whose opposite sides contacts 2 and 3am provided. Said plate 1 ismade from a diamondinwhich the distance traveled by the electrons islonger than that traveled by the holes. Therefore, the contact 2 is madefrom silver which, in conjunction with" diamond and under the influenceof the positive potential applied to it, injects holes into the diamondcrystal. The opposite contact 3 is made from gold and is blocking inrelation to the charge carriers when a negative potential is applied toit.

Nuclear radiation entering the detector from the side of the blockingcontact 3 causes ionization inside the diamond crystal. The resultingcharge carriers, i.e. electrons and holes, move to the respectivecontacts under the influence of the applied field, the electrons movingto the contact-2, and the holes traveling to the contact 3. Thethickness of the crystal plate 1 "does not exceed the distance traveledby the charge carriers in the diamond crystal under the influence of theapplied field. In thecase of detectors operating with complete chargecollection, high energy resolution and good counting efficiency, thefollowingcondition should be observed,

where t is the mobility of the charge carriers, vr-is the life of thecharge carriers, E is the applied field strength, 8 is the distancetraveled by the charge carriers under the influence of the appliedfield, and d is the thickness of the diamond crystal plate.

It is well known that in diamonds in which the nitrogen concentrationdetermined by optical absorption at a wavelength of 7.8 p. is less than10 atoms cm', the mobility of electrons is about 2,000 cm V sec' at roomtemperature, while the mobility of holes is about 1,500 cm V sec. In thepurest diamond crystals, the lifetime of the charge carriers ranges from10 tolO' sec. Experiments have shown that the mobility of electrons andholes in diamond at high electric fields decreases as the field isincreased, at first proportional to E then proportional to E beginningwith a field strength of 10 V cm for electrons at room temperature.Thus, the drift velocity ,u E saturates at high fields and its limit forelectrons is 10 cm sec at room temperature. Consequently, at a lifetimeof the charge carriers of 10' sec., the diamond crystal plate for thedetector operating with complete charge collection has an optimumthickness of 0.2 to 0.3 mm. At the shorter lifetime of the chargecarriers, the diamond crystal plate should be thinner and its thicknessis estimated in accordance with the above equation.

During their movement to the contact 2, some electrons are trapped bytraps always present in the crystal. As a result, the diamond crystalplate polarizes. The injecting contact 2 is designed to remove saidpolarization. Since deep traps are present in a diamond, the injectioncurrents from the contact 2 are limited by a space charge accumulated bysaid traps. Thus, the injection currents do not induce significantconductivity and, consequently, noise. However, when field and chargeequilibrium inside the crystal is disturbed due to polarization createdby incident nuclear radiation,

charge emission from the-contact 2 restores the initial 4 steady stateof the crystal. Since the higher field strength withinthe ionizationzone favors the reduction of losses in the electron-hole plasma whenusing the detector for counting the nuclear particles with lowpenetration, the blocking contact 3 should be located on the irradiatedside of the plate 1.

The charge carriers, i.e. holes, which move to the blocking contact 3under the influence of the applied field may also be trapped. In thiscase, however, the trapped holes are in the ionization zone and can beneutralized by the charge carriers of opposite sign, i.e.

'by-the electrons. I g Thus, the present detector operates with completecharge collection and does not polarize under prolonged irradiation dueto the vfact that the thickness of the diamond crystal plate does notexceed the distance traveled by the charge carriers and the appropriatecontact system is provided.

In like manner,-the detector can be manufactured from a diamond crystalin which the distance traveled by electrons is less than that traveledby holes. The differenceis that the contact on the irradiated side is anblocking in relation to electrons and a'positivepotential is applied toit, while the opposite contact injects electrons and the negativepotential is applied to it.

The detector shown in FIG. 2 is made from a diamond crystal plate whosethickness is considerably larger than the distance traveled by thecharge carriers. A recess is therefore in the crystalplate, thethickness of the bottom of said recess not exceeding the distancetraveled by the charge carriers. This detector operates in the same wayas the detector described above, has greater mechanical strength owingto the thickened peripheral area and is more convenient with respect tohandling.

The diamond detector described hereinabove is made from natural diamondwith a nitrogen contact less than 10 atoms cm. The selection of crystalsfor making detectors is based on the estimation of the mean lifetime ofthe charge carriers throughout the crystal by measuring a photocurrentvalue at a wavelength of 250 mg. The photoconductivity at 250 mp. isimperfection photoconductivity and the optical absorption coefficent at250 mu has a value ranging from 5 to 15 cm. Under these conditions onemay consider, with an accuracy of 50 percent, that the light iscompletely absorbed in said crystals. Then, the mean lifetime of thecharge carriers throughout the crystal can be estimated on a value ofphotocurrent, without measuring the absorption coefficient. Crystalswith the mean lifetime longer than 10' sec are selected for making thedetectors.

The selected crystals are cut into plates 0.1 to 0.3 mm thick. Aftercutting, the plates are placed in a 10 torr vacuum and annealed for 6 to8 hours at a temperature ranging from 1,000 to 1,300C. In some cases,the thermal treatment increases the lifetime of the charge carriersconsiderably.

In the annealed plates, the lifetime of the charge carriers is once moreestimated by using the same technique, but at wavelengths of 225 and 220mu. The edge absorption begins in diamonds at these wavelengths. Theabsorption coefficients are 20 and 1,000 cm, respectively. The meanlifetime of the charge carriers throughout the crystal is estimated at avalue of photocurrent at 225 mp, whereas the effect of surfacerecombination upon the value of photoconductivity is estimated at awavelength of 220 my. (the depth of light penetration is about 20 M).

Plates with a mean lifetime throughout the crystal of the order of secand higher are selected for further treatment.

In some cases, surface recombination is significant. The reduction ofthe rate of surface recombination is obtained by oxygen etching thespecimens in the atmosphere for several minutes at a temperature of 800to 900C. If the thickness of the diamond plate after cutting, annealingand etching is larger than necessary for the operation of the detectorwith complete charge collection, the plate is reduced to the desiredthickness, for example, by polishing, grinding or etching.

After mechanical treatment, the crystal plate is subjected to thermaltreatment as it, has been described hereinabove and to etching.

Then, contacts are applied to the prepared plate. The simplest method ofobtaining a contact for injecting holes on one side of the plateconsists of applying silver paint with subsequent burning of the paintinto the plate in the atmosphere at about 600C for 2 to 3 hours. Ablocking contact is formed by evaporating a film of gold over theopposite side of the plate in vacuum at room temperature.

In like manner, an injecting contact can be formed on one side of theplace by burning onto the plate gold or platinum from a paint. A contactfor injecting holes is also formed by restoring platinum, silver or goldfrom a solution of their salts by heating the diamond crystal platecovered with said solution to a temperature in the range of from 500 to700C for several minutes.

For the formation of graphite contact for injecting both electrons andholes, a colloidal graphite suspension, such as Aquadag, is applied toone side of the diamond crystal plate and the plateis heated to atemperature ranging from 500 to 600C in a vacuum for about 3 hours.Then, over the opposite side of the plate, a film of gold is evaporatedfor forming a blocking contact.

In some cases, a blocking contact is obtained by graphitizing thediamond crystal plate by heating the same in the temperature range offrom l,000 to 1,300C in a vacuum of 0.1 torr for about 30 min. Then, theresulting graphite layer is removed from one side of the plate. A filmof a colloidal graphite suspension is applied to this side. Then theplate is heated in a vacuum at a temperature ranging from 500 to 600Cfor obtaining an injecting contact. In like manner, an injecting contactcan be obtained after the removal of said layer by burning into theplate silver from a paint as described above. The present diamonddetector for nuclear radiations has a number of advantages. It candetect nuclear particles with the range up to 2X10 cm and operates atroom and higher temperatures. In addition, it possesses a high energyresolving power of 7 percent at room temperature and a countingefficiency of 100 percent. The detector operates with complete chargecollection and does not polarize under prolonged irradiation.

We claim:

1. A method of manufacturing a nuclear radiation detector, comprisingforming a plate from a diamond crystal, the thickness of which platedoes not exceed the distance traveled by charge carriers under theinfluence of an applied field, annealing said plate in a vacuum at atemperature of l,000 to 1,300C, and then forming blocking and injectingcontacts on opposite sides of said plate so that during operationpolarization within the plate can be removed by injection of chargecarriers into said plate via the injecting contact, said injectingcontact being formed on the side of the plate opposite to that to besubjected to the radiation to be detected, said injecting contact beingformed of a ma terial capable, in conjunction with the diamond crystalplate, of injecting the charge carriers under the influence of saidfield,

2. A method of manufacturing a detector according to claim 1, wherein,prior to forming the contacts, the annealed diamond crystal plate isetched by heating the same in an oxygen-containing medium for decreasingthe rate of surface recombination of the charge carriers.

3. A method of manufacturing a detector according to claim 1, whereinboth sides of the diamond crystal plate are covered with silver paint,then the plate is heated to a temperature of 500 to 700C until silvercontacts are formed on the sides of the plates.

4. A method of manufacturing a detector according to claim 1, whereinboth sides of the diamond crystal plate are covered with gold paint,then the plate is heated to a temperature of 500 to 700C until goldcontacts are formed on both sides of the plate.

5. A method of manufacturing a detector according to claim '1, whereinboth sides of the diamond crystal plate are covered with platinum paint,then the plate is heated to a temperature of 500 to 700C until platinumcontacts are formed on both sides of the plate.

6. A method of manufacturing a detector, according to claim t,whereinboth sidesofthe diamond crystal plate are covered with a solution ofilver au's', then the plate is heated to a temperature of 500 to 700Cuntil silver is deposited for forming the contacts.

7. A method of manufacturing a detector according to claim 1, whereinboth sides of the diamond crystal piatear'e covered Witfi'a sbltftitjriOf g CSTd sansithen the plate is heated to a temperature of 500 to 700Cuntil gold is deposited for forming the contacts.

8. A method of manufacturing a detector, according to claim 1, whereinboth sides of the diamond crystal plate are covered with a solution ofplatinum salts, then the plate is heated to a temperature of 500 to 700Cuntil platinum is deposited for forming the contacts.

9. A method of manufacturing a detector according t o claim 1, whereinthe blocking contact is first formed on one side of the diamond crystalplate, whereupon a colloidal graphite suspension is applied to theopposite sides and the plate is heated to a temperature of 500 to 600Cin vacuum for forming the injecting contact.

10. A method for manufacturing a detector according to claim 1, whereinthe diamond crystal plate is graphitized for" forming the'bmazfirg'arfiaa, whereupon a resulting graphite layer is removed fromone side of the plate, a colloidal graphite suspension is applied to itand the plate is heated to a temperature of 500 to 600C in vacuum forforming the injecting contact.

11. A method of manufacturing a detector according to claim 1, whereinthe diamond crystal plate is graphitized for forming"theblockingcontactf whereupon a resulting graphite layer is removed from one sideof the plate, silver paint is applied thereto, and the plate is heatedto a temperature of 500 to 700C in vacuum until a silver injectingcontact is formed thereon.

1. A method of manufacturing a nuclear radiation detector, comprisingforming a plate from a diamond crystal, the thickness of which platedoes not exceed the distance traveled by charge carriers under theinfluence of an applied field, annealing said plate in a vacuum at atemperature of 1,000* to 1,300*C, and then forming blocking andinjecting contacts on opposite sides of said plate so that duringoperation polarization within the plate can be removed by injection ofcharge carriers into said plate via the injecting contact, saidinjecting contact being formed on the side of the plate opposite to thatto be subjected to the radiation to be detected, said injecting contactbeing formed of a material capable, in conjunction with the diamondcrystal plate, of injecting the charge carriers under the influence ofsaid field.
 2. A method of manufacturing a detector according to claim1, wherein, prior to forming the Contacts, the annealed diamond crystalplate is etched by heating the same in an oxygen-containing medium fordecreasing the rate of surface recombination of the charge carriers. 3.A method of manufacturing a detector according to claim 1, wherein bothsides of the diamond crystal plate are covered with silver paint, thenthe plate is heated to a temperature of 500* to 700*C until silvercontacts are formed on the sides of the plates.
 4. A method ofmanufacturing a detector according to claim 1, wherein both sides of thediamond crystal plate are covered with gold paint, then the plate isheated to a temperature of 500* to 700*C until gold contacts are formedon both sides of the plate.
 5. A method of manufacturing a detectoraccording to claim 1, wherein both sides of the diamond crystal plateare covered with platinum paint, then the plate is heated to atemperature of 500* to 700*C until platinum contacts are formed on bothsides of the plate.
 6. A method of manufacturing a detector, accordingto claim 15, wherein both sides of the diamond crystal plate are coveredwith a solution of silver salts, then the plate is heated to atemperature of 500* to 700*C until silver is deposited for forming thecontacts.
 7. A method of manufacturing a detector according to claim 15,wherein both sides of the diamond crystal plate are covered with asolution of gold salts, then the plate is heated to a temperature of500* to 700*C until gold is deposited for forming the contacts.
 8. Amethod of manufacturing a detector, according to claim 15, wherein bothsides of the diamond crystal plate are covered with a solution ofplatinum salts, then the plate is heated to a temperature of 500* to700*C until platinum is deposited for forming the contacts.
 9. A methodof manufacturing a detector according to claim 15, wherein the blockingcontact is first formed on one side of the diamond crystal plate,whereupon a colloidal graphite suspension is applied to the oppositesides and the plate is heated to a temperature of 500* to 600*C invacuum for forming the injecting contact.
 10. A method for manufacturinga detector according to claim 15, wherein the diamond crystal plate isgraphitized for forming the blocking contact, whereupon a resultinggraphite layer is removed from one side of the plate, a colloidalgraphite suspension is applied to it and the plate is heated to atemperature of 500* to 600*C in vacuum for forming the injectingcontact.
 11. A method of manufacturing a detector according to claim 1,therein the diamond crystal plate is graphitized for forming theblocking contact, whereupon a resulting graphite layer is removed fromone side of the plate, silver paint is applied thereto, and the plate isheated to a temperature of 500* to 700*C in vacuum until a silverinjecting contact is formed thereon.